Studies On The Mechanisms Of Improvement Of Cadmium Tolerance By Arbuscular Mycorrhizal Fungi Inoculation In Soybean

The excessive accumulation of cadmium(Cd)in soils is one of the major limiting factors for the development of agricultural production.Soybean is an important cereal and oil crop.Soybean production is seriously limited by excessive Cd accumulation in the soils of South China.Plants have evolved a series of mechanisms to cope with Cd toxicity under cadmium stress conditions.The symbiosis with arbuscular mycorrhizal fungi(AMF)is an important way to improve Cd tolerance in plants.Whether do soybean genotypes with different Cd tolerance show differential response to AMF inoculation,and whether does AMF inoculation improve Cd tolerance in soybean,and what are the underlying mechanisms on it.There is little report on these aspects.In the present study,culture medium and sand/soil pot experiments were carried out to study the effects of different Cd concentration and inoculation treatments on plant growth,root traits,nutrient element distribution in plants,and gene expression using four different soybean genotypes,and thereby explore the molecular and physiological mechanisms of improvement of Cd tolerance by AMF inoculation.The main results are as follows:(1)Cadmium toxicity severely inhibited soybean growth.Soybean seed germination experiments showed that Cd significantly inhibited tap root growth.The tap roots hardly grew at 500 ?M Cd concentration.Cadmium toxicity not only affected root growth,but also seriously affected shoot growth of different soybean genotypes.There were significant genotypic differences in Cd tolerance in four soybean genotypes.Huaxia No.3(HX)and Guixiadou No.2(GX)were more tolerant to cadmium stress,while Baxi No.10(BX)and Bendi No.2(BD)were more sensitive to cadmium stress.(2)AMF inoculation can promote soybean growth.There were significant differences in AMF inoculation effect on different soybean genotypes at different Cd treatments.Under no Cd conditions,the biomasses of BX,GX and HX increased significantly after inoculation with AMF.Under the conditions of 10 mg/kg Cd and 20 mg/kg Cd,only Cd-tolerant genotype HX increased significantly biomass when inoculated with AMF.Compared to no inoculation treatments,the biomasses of HX inoculated with AMF increased by 44% and 17% at 10 mg/kg Cd and 20 mg/kg Cd treatments,respectively.In addition,AM colonization of HX was significantly higher than those of other genotypes at 10 mg/kg Cd treatment.These suggest that AMF inoculation can further help Cd-tolerant HX to alleviate Cd toxicity.(3)Phosphorus(P)contents of shoots and roots of inoculated Cd-tolerant HX were significantly higher than those of non-inoculated plants under the conditions of 10 mg/kg Cd and 20 mg/kg Cd.Correspondingly,the expression levels of mycorrhizal-induced phosphate transpoters GmPT8,GmPT9,and GmPT10 were significantly up-regulated in HX roots inoculated with AMF compared to those of non-inoculated plants,indicating that the inoculated HX plants acquired and transported P mainly through the mycorrhizal P uptake pathway,which can remedy weakened root absorption capacity by Cd toxicity.(4)In addition to P acquisition,there were significant differences in the other nutrient element concentrations in plants between Cd-sensitive and Cd-tolerant genotypes.In the shoots,the K and Zn concentrations of Cd-sensitive BX plants significantly changed after inoculation with AMF compared to the non-inoculated plants,whereas the Cu and Mn concentrations of Cd-tolerant HX plants changed significantly.In the roots,the Mg concentration of BX plants changed significantly,while the Cu and Mn concentrations of HX plants changed significantly after AMF inoculation compared with the non-inoculated plants.It indicates that the AMF inoculation might regulate the changes of nutrient element concentrations in plants,and thereby help the plants alleviate Cd toxicity.(5)The culture medium experiments of hairy roots symbiosis with arbuscular mycorrhizal fungi showed that the mycelia grown in the medium containing Cd had strong fluorescence under confocal microscopy observation,indicating that Cd can be directly adsorbed and fixed into mycelia by AMF,and thereby Cd absorption by plants reduces,and AMF inoculation can help plants to alleviate Cd toxicity.In summary,there were significant genotypic differences in Cd tolerance in soybean.BX and BD were Cd-sensitive genotypes,and GX and HX were Cd-tolerant genotypes.AMF inoculation can further alleviate Cd toxicity in Cd-tolerant HX through acquiring P by the mycorrhizal P uptake pathway,and changing the concentrations of other nutrient elements in plants.In addition,AMF mycelia can directly adsorb Cd and reduce the absorption of Cd by plants.The results will provide a theoretical basis for improving Cd tolerance through the mycorrhizal pathway in soybean.